1. Field of the Invention
The present invention relates to a radio frequency device, more particularly to a radio frequency device that has an input transistor that sees a capacitance that needs to be charged when the radio frequency device is switched from off to on, and/or to a higher gain mode. Such a radio frequency device can be a low noise amplifier, a mixer, or any other suitable radio frequency device. The radio frequency device is used in a receiver, a transmitter, or transceiver, or the like.
2. Description of the Related Art
Gain-mode switchable devices such as low noise radio frequency amplifiers are known. Such devices have at least two switchable gain paths, a high gain path and a low gain path. Particularly in such systems as CDMA mobile radio systems, but also in many other systems, low noise radio frequency amplifiers should be highly linear. A key parameter to express linearity is the so-called Input Third Order Intercept Point (IIP3) of the amplifier. This is a measure for the third order distortion of the amplifier. The IIP3-point is defined as the point where the linearly extrapolated fundamental frequency characteristic of the amplifier intercepts the third order characteristic of the amplifier, i.e., has the same output power at the same input power, when two tones of a different frequency are applied to the input of the amplifier, i.e., when so-called xcex94f modulation is applied. In a typical CDMA application the radio frequency is 2 GHz and the test tones differ 1 MHz. CDMA systems such as in the USA, share their frequency band with the so-called AMPS system. Under such circumstances, the IIP3 parameter becomes a critical radio frequency parameter because large AMPS signals in the neighborhood of a wanted CDMA signal could work as blocking signals when the low noise amplifier is not sufficiently linear. One measure to extend a linearity range of a low noise radio frequency amplifier is to degenerate an emitter path of an input transistor of the low noise radio frequency amplifier by putting resistance and/or inductance between the emitter of the transistor and an output pin of an integrated circuit embodying the radio frequency device. Other key parameters of the amplifier are its gain and noise figure. In this respect, a better linearity has a degrading effect on the noise figure so that a trade-off has to be made. In order to improve linearity of the low noise radio frequency amplifier, a capacitor to ground, that is large for the application, is preferably connected to the input electrode of the input transistor. Such a large capacitor is typically in a range of 100 pF to 20 nF and forms a low impedance at the xcex94f modulation. The input electrode of the input electrode is typically coupled to a matching network, such as a 50xcexa9 matching network, via a DC-blocking capacitor. In addition thereto, parasitic capacitance is present at the input electrode of the input transistor. The low noise radio frequency amplifier is switchable from a low gain where the voltage at the input electrode is close to zero volts to a high gain mode where the voltage at the input electrode is finite. Low/high gain mode can be implemented by providing a cascode transistor arrangement wherein the input transistor is configured as a common emitter amplifier that is switched on in the high gain mode, and off when the amplifier is in the low gain mode. In low gain mode an alternative low gain path would be active. Proper biasing of the input transistor causes the input transistor to turn off. Such a cascode transistor arrangement is applied in an Integrated Circuit of type SA2421. Such a mode-switching is important when using the radio frequency device in portable transceivers like mobile radio handsets, so as to increase the stand-by time of the transceiver. In order not to loose data, it is important that a so-called switching time to switch the low noise radio frequency amplifier from a low to a high gain is small. The application of a large capacitance at the input electrode of the input transistor tends to increase the switching time because it takes too long for the large capacitor to charge it to the required turn on voltage, such a charging occurring xe2x80x98naturallyxe2x80x99 through the biasing.
It is an object of the invention to provide a highly linear gain-mode switchable radio frequency device that has a short switching on time when being switched from a low gain to a high gain mode.
It is another object of the invention to provide a single gain mode radio frequency device that has a short switching on time when being switched from off to on.
It is another object of the invention to provide such a radio frequency device wherein the input noise figure is hardly affected.
It is still another object of the invention to provide a radio frequency device with an automatic input capacitance charging shut-off mechanism.
In accordance with the invention, a radio frequency device is provided, said radio frequency device comprising:
an input transistor comprising an input electrode from which a capacitance is seen, said input transistor being switchable between an on-mode and an off-mode, and said input electrode receiving a radio frequency signal; and
a charging circuit for charging said capacitance when said input transistor is switched from said off-mode to said on-mode.
In an embodiment, the radio frequency device further comprises gain mode switching means for switching said radio frequency device into at least a first gain mode and a second gain mode, in said first gain mode said radio frequency device having a lower gain than in said second gain mode, said input transistor being coupled to said gain mode switching means, and said gain mode switching means switching said radio frequency device from said first into said second gain mode when said input transistor is switched from said off-mode to said on-mode.
The invention is based on the insight that, by not relying on natural charging through biasing alone of the input transistor, when being switched from off to on, and/or from a low to a high gain mode, but by providing a charge boost to charge the input capacitance seen by the input transistor, the switching time can be greatly reduced.
In a very advantageous embodiment of the invention, the quantity or variable defining switching off of the charging boost mechanism is a quantity not measured at the input electrode of the input transistor but a quantity at another node. Herewith, the input noise figure of the radio frequency device is minimally affected. Such a quantity can be a current flowing through a resonance suppression resistor in series with a bond wire through which DC power is provided to the radio frequency device. When the radio frequency device switches to the on-mode and/or to the high gain mode, after the input capacitance has been charged to a sufficient level the input transistor carries a current that is detectable at the output of the radio frequency device. Such detection is used to automatically shut-off charging.
In an advantageous embodiment of the invention, a high charging current is obtained through current amplification via a charging transistor. In order to minimize a load effect on the input transistor, the charging transistor is fully shut-off once charging of the capacitance is completed. Herewith, the charging process imposes minimal impact upon the input noise figure of the radio frequency device.